1. Field of the Invention
The present invention relates to devices for monitoring electrical circuits and, more particularly, to devices for detecting undervoltage conditions on any phase of multiphase alternating current circuits.
2. Description of the Prior Art
Many types of commercial and industrial activity depend on electrical equipment operating from multi-phase alternating current circuits. This equipment is usually designed for operation at a specified voltage and may sustain extensive damage from operation on circuits supplying less than the specified voltage. In order to prevent damage of this type it is necessary to detect undervoltage conditions on the supply circuit and to take action to correct the problem. Desirable corrective action may include actuating an alarm indicating that an undervoltage condition exists or switching from the normal power supply to an emergency power supply.
In order to fully protect equipment from damage due to undervoltage conditions it is necessary to monitor all phases of a multi-phase circuit. This has been accomplished in the past by monitoring each individual phase with an individual device. Examples of such single phase monitoring devices are set forth in U.S. Pat. Nos. 2,959,717 and 3,718,839. Use of individual monitoring devices for each phase is costly however and it would be desirable to utilize a single device to monitor voltage conditions on all phases of a multi-phase circuit.
Monitoring devices are normally designed to actuate, or "drop out", when the voltage on a circuit being monitored falls below a certain value. If the voltage being monitored should oscillate within a small range around this dropout voltage, the monitoring device may alternately actuate and deactuate, a condition known as "chattering". This condition is undesirable for it may generate multiple alarms or repeated switchover between the normal and the emergency power supply. It is therefore desirable to provide a differential between pickup and dropout voltages of the monitoring device.
Faults occurring on the power system supplying the circuits being monitored can result in rapid multiple reclosing of circuit breakers, causing alternate normal-abnormal conditions on the circuit being monitored. This also could result in multiple alarms and rapid switchover between normal and emergency supply circuits. It is desirable for a monitoring device to provide capability to ignore short transient normal conditions and to pick-up, that is, to indicate a return to normal following an abnormal condition, only after a specified time delay. It is also desirable for a monitoring device to provide capability to vary the dropout voltage with a fine degree of control.